To convert tonnes to kgs multiply by 1000. I've generated some hopefully helpful examples randomly below, as with most random sentences please excuse the repetitive and occasionally comic outcomes.
You have 31 tonnes of tin but you need to know how many kgs that is. To get the answer multiply by 1000 which gives you 31000 kgs.
You've got 60 tonnes of steel but someone only wants to buy that in units of kgs, to change the units multiply by 1000. Now you have 60000 kgs.
How many kgs of lead are there in 57 tonnes? Just multiply by 1000 to get the answer of 57000 kgs.
To convert 81 tonnes into kgs multiply by 1000 giving 81000 kgs.